1. Field of the Invention
The present invention relates to a mass spectrometer system and mass spectrometry method and, more particularly, to an ion attachment mass spectrometry technique of analyzing the mass of neutral molecules having metal ions attached to them.
2. Description of the Related Art
IAMS (Ion Attachment Mass Spectrometry) is mass spectrometry which ionizes neutral gas phase molecules (gas) without dissociating (fragmenting) them (molecular ions) and analyzes the mass of the molecular ions. This method is effective for analysis of an organic material which readily causes decomposition (dissociation, fission, or fragmentation) upon ionization.
Non-patent references 1 to 5 describe ion attachment mass spectrometer systems. Non-patent references 6 and 7 describe the influences of temperatures on ion attachment mass spectrometer systems.
FIG. 7 is a view showing an example of the arrangement of an ion attachment mass spectrometer system (to be abbreviated as a mass spectrometer system hereinafter) for a solid/liquid sample. Referring to FIG. 7, an ion generation source 100 and a sample vaporization chamber 140 are arranged in a first cell 180. A mass analyzer 160 is arranged in a second cell 190. A vacuum pump 170 reduces the pressure in the first cell 180 and the second cell 190. Hence, all the ion generation source 100, sample vaporization chamber 140, and mass analyzer 160 exist in a low-pressure atmosphere having a pressure lower than the atmospheric pressure.
The ion generation source 100 includes an emitter 120 serving as an ion-emitting unit in a chamber 110. The emitter 120 is a sintered body made of an alumina silicade (an eutectic material of aluminum oxide and silicon oxide) containing an oxide, carbonate, or salt of an alkali metal (e.g., Li) etc. When heated to about 600° C. to 800° C. in a low pressure atmosphere, the emitter 120 generates, from its surface, positively charged alkali metal ions (metal ions) such as Li+. A solid/liquid sample 150 is heated in the sample vaporization chamber 140 serving as a neutral molecule introduction means so as to turn into neutral gas phase molecules (gas), that is, vaporized neutral molecules. The neutral gas phase molecules then move to the ion generation source 100 by, for example, diffusion, gas flow, or buoyancy of themselves and enter the chamber 110.
Next, the ion generation source 100 ionizes the neutral gas phase molecules to generate molecular ions. The metal ions attach to the charge localized portions of the neutral gas phase molecules. The molecules with the metal ions attached (ion-attached molecules) form ions having positive charges as a whole.
However, after metal ion attachment to the neutral gas phase molecules, if the ion-attached molecules are left as they are (keep holding the extra energy), the extra energy dissociates the bond between the metal ions and the neutral gas phase molecules. When the metal ions are separated from the neutral gas phase molecules, the ion-attached molecules return to the original neutral gas phase molecules. To prevent this, a gas such as N2 is introduced into the ion generation source 100 at a pressure of about 50 to 100 Pa (at a flow rate of 5 to 10 sccm) to cause the ion-attached molecules to often collide with the gas molecules. At this time, the extra energy held by the ion-attached molecules moves to the other gas molecules, and the ion-attached molecules stabilize. The other gas is called a three-body gas. A three-body gas cylinder 200 serving as a three-body gas introduction means is connected to the ion generation source 100 via a pipe to introduce the gas into the chamber 110.
The effect of the three-body gas will be explained here with reference to FIG. 9. FIG. 9 shows the potential energy near the ion-attached molecules. Reference numeral 801 indicates a potential near the molecules; and 802, an ion such as Li+ attached to the molecules. Since the potential 801 has a potential well as shown in FIG. 9, the ion 802 oscillates around a lowest point 803 of the potential. However, when a three-body gas such as nitrogen collides with the ion-attached molecules, the oscillation energy moves to the three-body gas so that the molecules can stably continue existing in the ion-attached state. As a result, the molecules are ionized without being fragmented. That is, molecular ions in the original molecular state are formed.
An ion attachment region 210 where the metal ions attach to the neutral gas phase molecules can be limited to a region where the metal ions emitted from the emitter 120, the neutral gas phase molecules corresponding to the sample component, and the three-body gas introduced from the outside exist simultaneously.
Finally, the ion-attached molecules are transported from the ion generation source 100 (communicating hole 110a of the chamber 110) to the mass analyzer 160 upon receiving the force of an electric field. The mass analyzer 160 fractionates and measures the mass of the ions. To generate the electric field, the potential of the entire ion generation source 100 is set to be positive (e.g., 10 V), and the potential of the entire mass analyzer 160 is set to 0 V most commonly, although not illustrated.
The ion attachment mass spectrometry capable of ionizing original molecules without decomposing them is advantageous because it enables highly accurate, quick, and simple measurement, as will be explained below.
In techniques other than the ion attachment mass spectrometry, various kinds of decomposition peaks appear in a mass spectrum. It is therefore necessary to separate components using a gas chromatograph (GC) or a liquid chromatograph (LC) before mass analysis. To normally separate the components of many samples by GC/LC, a complex and cumbersome preprocess is required for each sample. Normally, the component separation takes several ten min, and the preprocess takes several to several tens of hours.
On the other hand, a mass spectrum measured by the ion attachment mass spectrometry has no decomposition peak but only the original molecular peak. In short, a sample containing n kinds of components exhibits n peaks, and the components can be qualitatively and quantitatively measured based on their mass numbers. It is therefore possible to directly measure even a mixed sample containing a plurality of components without component separation. The ion attachment mass spectrometry requires neither preprocess nor component separation necessary in other techniques. Hence, measurement can end in only several minutes, and highly accurate, quick, and simple measurement can be done.
FIG. 8 illustrates the arrangement of another conventional ion attachment mass spectrometer system for a gas sample. The same reference numerals as in FIG. 7 denote the same parts in FIG. 8. Since the sample is gaseous, no sample vaporization chamber 140 exists. The sample is directly introduced from a sample gas cylinder 220 to the ion generation source 100. The remaining structures, operations, and advantages in measurement are the same as in FIG. 7.
Prior-art references are Japanese Patent Laid-Open Nos. 6-11485, 2001-174437, 2001-351567, 2001-351568, 2002-124208, 2002-170518, and 2002-298776.
Non-patent reference 1 is “Hodge (Analytical Chemistry vol. 48, No. 6, p. 825 (1976))”. Non-patent reference 2 is “Bombick (Analytical Chemistry vol. 56, No. 3, p. 396 (1984))”. Non-patent reference 3 is “Fujii (Analytical Chemistry vol. 61, No. 9, p. 1026 (1989))”. Non-patent reference 4 is “Chemical Physics Letters vol. 191, No. 1.2, p. 162 (1992)”. Non-patent reference 5 is “Rapid Communication in Mass Spectrometry vol. 14, p. 1066 (2000)”. Non-patent reference 6 is “Analytical Chemistry, vol. 53, p. 475 (2004)”. Non-patent reference 7 is “Vacuum, vol. 50, p. 234 (2007)”.
In the ionization method using the above-described ion attachment method, if the ion generation source is warmed to general 150° C. to 200° C. to reduce the influence of condensation/adsorption at the ion generation source, the ionization efficiency (sensitivity) greatly lowers in some substances on one hand, and the influence of condensation/adsorption remains in other substances on the other hand.